1. Field of the Invention
The present invention relates to systems for retrieval wellbore fluids. More particularly, the present invention relates to systems and devices for transporting return fluids from a seabed to a location on a water surface. In a different aspect, the present invention relates to methods for conveying drill fluids from a seabed to a surface location.
2. Description of the Related Art
Conventional hydrocarbon recovery operations typically include a derrick disposed over a subterranean formation bearing oil and gas deposits. For offshore hydrocarbon recovery operations, the derrick is erected on a platform at the water surface. A drill string suspended from the derrick includes a drill bit adapted to disintegrate earth and rock and thereby form a wellbore. Often, a riser extending from the platform to a subsea wellhead at a seabed or mud line is used to guide the drilling string into the formation of interest. The drill pipe or drill string can include a plurality of joints of pipe or coiled tubing, each of which has an internal, longitudinally extending bore for carrying drilling fluid from the well drilling platform through the drill string and to a drill bit. Drilling fluid lubricates the drill bit and carries away well cuttings generated by the drill bit. The cuttings are carried in a return flow stream of drilling fluid through the well annulus and is either recovered or dumped.
In some instances, the seabed has a relatively deep layer of soft sediment or earth. This soft layer can pose difficulties during offshore well construction because it is ill suited to support the heavy equipment and structures that are installed at the seabed to support drilling activities. One conventional method used to overcome this problem is to drill, case and cement a relatively deep large diameter well bore (e.g., thirty to thirty-six inch diameters). This casing thereafter provides the needed foundation for a wellhead and for hanging or supporting well head equipment. Once this casing is set the next string of casing, normally a 20″ diameter surface casing, requiring that a 26″ diameter hole is drilled and set using the same procedure. As can be appreciated, the drilling of such a relatively large diameter wellbore requires the removal of a substantial amount of earth and rock. Thus, a significant amount of drilling fluid is needed to flush out and convey the drill cuttings to the seabed. In certain instances, water or seawater can be used as the drilling fluid for lubricating the drill bit and removing cuttings. The returning seawater is often simply released to the marine environment in the vicinity of the wellbore. In other instances, however, seawater is not adequate to promote safe and or efficient drilling due to either insufficient pressure or weight. In these instances, drilling “mud” is used as the drilling fluid. The returning drilling “mud,” like seawater, is also released at the seabed because during this operation the riser is not yet in place and conventional methods for recovering return fluid are not cost-effective. For example, a seabed-based pump having the capacity to pump the return fluid to the platform would be very expensive to deploy and operate.
Thus, there is a need for more cost-effective and efficient method of recovering the drilling mud used during this offshore drilling operation. In addition, the environmental impact of such operations would be improved immensely if the mud were to be recovered rather circulated into the sea.